Effects Of Long-term Nitrogen Addition And Biochar Amendment On Greenhouse Gases Fluxes In Moso Bamboo Plantations

Increasing emissions of greenhouse gases(GHGs),especially carbon dioxide(CO₂),methane(CH₄),and nitrous oxide(N₂O),are the major drivers of global climate change.Forests are significant sources or sinks for GHGs,and thus play an important role in regulating global climate change.Atmospheric N deposition enhances soil N availability and often induces forest soil acidification,both of which can increase soil GHGs emissions.Biochar has the characteristic of alkaline and strong sorption capacity,which can decrease inorganic N pool and increase soil pH,then decreasing soil GHGs emissions.The application of biochar to cropland has been recommended as a strategy for reducing soil CO₂ emissions and mitigating climate change.However,it remains unclear whether biochar amendment can offset the positive effects of N deposition on GHG emissions in forest soils.Here,we investigated and quantified the separate and combined effects of different levels of N addition(Control,0 kg N ha^-1 yr^-1;N30,30 kg N ha^-1 yr^-1;N60,60 kg N ha^-1 yr^-1;N90,90 kg N ha^-1 yr^-1)and biochar amendments(BC0,0 t biochar ha^-1;BC20,20 t biochar ha^-1;and BC40,40 t biochar ha^-1)on soil GHG fluxes in a long-term(>5 years)field experiment at a Moso bamboo plantation in subtropical China.(1)Low and moderate N additions significantly increased annual soil CO₂ and N₂O emissions by 17.0%-25.4%and 29.8%-31.2%,respectively,but decreased CH₄ uptake by12.4%-15.9%,leading to a globe warming potential(GWP)increase of 17.9%-25.9%.Meanwhile,high N addition levels significantly increased annual soil N₂O emissions only by20.4%and decreased CH₄ uptake by 16.8%,but the high N treatment did not significantly affect CO₂ emissions or GWP.The promoting effect of N addition on soil CO₂ and N₂O emissions decreased with N addition time.N addition increased soil microbial biomass carbon(MBC),available N(AN),and NH₄⁺-N concentrations,but decreased soil pH.Soil CO₂ and N₂O fluxes were significantly and positively correlated with both soil MBC and AN concentration.Soil CH₄ flux was significantly and positively correlated with both soil MBC and pH,but was significantly and negatively correlated with NH₄⁺-N.(2)Biochar amendments(BC20 and BC40)significantly increased annual soil CO₂emissions and CH₄ uptake by 18.4%-25.4%and 7.6%-15.8%,respectively,but decreased N₂O emissions by 17.6%-19.2%,leading to a GWP increase of 16.4%-22.8%.Biochar amendment decreased soil MBC,AN,and NH₄⁺-N concentrations,but increased soil pH.Soil CO₂ flux was significantly and positively correlated with soil MBC.Soil N₂O flux was significantly and positively correlated with both soil AN and NH₄⁺-N concentrations but was significantly and negatively correlated with soil pH.Soil CH₄ flux was significantly and positively correlated with both soil MBC and pH,but was significantly and negatively correlated with NH₄⁺-N.(3)In N addition conditions,biochar amendments significantly enhanced the promoting effects of N addition on soil CO₂ emissions(11.2%-34.4%),but substantially offset the promoting effects of N addition on N₂O emissions(16.4%-35.9%)and the inhibiting effects of N addition on CH₄ uptake(9.7%-23.5%),leading to a GWP increase of 9.1%-30.3%.Moreover,the magnitude of these effects increased with the biochar amendment rate.N addition combined with biochar amendment decreased soil MBC,AN,and NH₄⁺-N concentrations,but increased soil pH compared to the N addition alone.Soil CO₂ flux was significantly and positively correlated with soil MBC,AN,and pH.Soil N₂O flux was significantly and positively correlated with soil MBC,AN,and NH₄⁺-N concentrations but was significantly and negatively correlated with soil pH.Soil CH₄ flux was significantly and positively correlated with both soil MBC and pH,but was significantly and negatively correlated with NH₄⁺-N.In summary,long-term(>5 years)N addition and biochar amendment significantly increased soil GHGs emissions,as calculated by the GWP approach.Furthermore,biochar amendment is not a recommended strategy for mitigation of soil GHG emissions under increasing atmospheric N deposition in Moso bamboo plantations,because biochar amendment does not offset the enhancement of GWP by elevated N deposition but instead further increases it.These results and findings could greatly deepen our understanding of the response patterns of soil GHGs emissions to N addition and biochar amendment and the associated mechanisms and provide a scientific basis for the adaptation mechanisms of forest ecosystems to the effects of N deposition and biochar amendment.It also could help managers and policy-makers to evaluate the ecological effects of environmental changes linked to human management practices.